In the past, power amplifiers for radio communication devices, such as radiotelephones, have used bipolar transistors. The simplest and most cost effective approach for building the amplifier would be to mount the bipolar transistor die in an industry standard plastic surface mountable package, and soldering the package to a circuit board.
A problem arose which made it impractical to use standard surface mount packaging in many power amplifier applications because bipolar transistors have an output signal and heat sink coming out of one side of the bipolar transistor die, and input signals and RF grounds coming out of the other side of the transistor die. The requirements of providing a low loss electrical path for the output signal and a thermally conductive path for heat dissipation on one side of the die, and low loss electrical paths for the input signal and electrically conductive paths for the RF grounds from the other side of the die necessitated the use of onboard die attachment process and a wirebond process to couple the transistors to a circuit board.
There is a requirement in the design of an amplifier to set the bias current at an optimal level for achieving, among other goals, the required gain, output power and efficiency. Additionally, there is a requirement in radiotelephone applications that the output power is controllable. For good power efficiency it is advantageous to control the output power by varying the bias current. The bias current is responsive to changes in a control voltage signal. The output power control range for each power amplifier stage is limited on the high end by the amplifier gain and on the low end by the isolation from input to output when the control voltage, and hence the bias current, is set to zero. Within this range the output power is approximately proportional to the square of the bias current. A potential problem exists when the bias current can not be held at its optimum value due to extreme sensitivity of bias current to changes in circuit parameters.
In a single stage amplifier, the sensitivity problem can be alleviated by using an output power control loop to automatically compensate for parameter shifts. However, only the final stage of a multi-stage bipolar amplifier could be controlled optimally by an output power control loop. Therefore, a multi-stage amplifier could not be properly biased in every stage at its optimum performance point if the circuit parameters are shifted. Additionally, under extreme parameter variations, the driver stage of the multi-stage amplifier could be damaged by over dissipation caused by too much bias current.
The bias current is exponentially-related to changes in the control voltage when using bipolar transistors. This exponential relationship caused potential problems due to high sensitivity of the bias current to small changes in circuit parameters. Particularly, the bias current is primarily sensitive to changes in the threshold voltage above which the bias current begins to flow. Fortunately, with bipolar transistors, because the thresholds were largely dependent on the built-in potential of a silicon pn junction, the bias current variation due to changes in this parameter was sufficiently low. However, the costly onboard die attach and wire bonding could not be eliminated.
It would be advantageous to have a single stage or a multi-stage amplifier employing devices which are easy to package, and having the output power controllable by varying the bias current, with low sensitivity of the bias current to control signal threshold variations.